In this paper, the sense and antisense mammalian expression vectors of T24-ras activated oncogene were introduced into cultured NIH3T3 cells in sense-->antisense and antisense-->sense orders. The transfected cells were selected by drug selection. The morphology change, Southern hybridization, frequency of soft agar colonies and the detection of P21 amount were investigated. The results showed that the sense recombinant could cause the malignant transformation of NIH3T3 cells while the antisense recombinant could inhibit this kind of transformation. Furthermore, the inhibitory effect varied according to the different transfected orders.